Integrated circuits are produced from wafers of semiconductor material. The wafers are typically housed in a cassette, that may be held in a carrier (e.g. FOUP, SMIF, etc.), having one or more closely spaced slots (as specified in SEMI standard), each of which can contain a wafer. The cassette is typically moved to a processing station where the wafers are removed from the cassette, placed in a predetermined orientation by a prealigner or otherwise processed, and returned to another location for further processing.
Various types of wafer handling devices are known for transporting the wafers to and from the cassette, within processing stations, and between processing stations. Many employ a robotic arm having an end effector that is inserted into the cassette to remove or insert a wafer. The end effector generally enters the cassette through the narrow gap between a pair of adjacent wafers and engages the backside of a wafer to retrieve it from the cassette. It is desired that, the end effector be thin, rigid, and positionable with high accuracy to fit between and not touch the closely spaced apart wafers in the cassette. After the wafer has been put through one or more processing steps, the robotic arm places the wafer back into the cassette.
Various means have been devised for centering and capturing a silicon wafer on the end effector. Some capture systems are active systems with one or more actuable components on the end effector. In one type of end effector, a series of vacuum suction pads or cups releasibly hold and center the wafer to the end effector. The suction pads or cups touch the back side of a wafer to mechanically secure it during handling. Touching the back side of the wafer is undesired as it may result in particulate matter contaminating the back of the wafer, and possibly falling onto processed surfaces of other stacked wafers (e.g. the transport). In another type of end effector, mechanical actuation through the use of hydraulically or pneumatically actuated arms or fingers are used to mechanically secure a wafer during handling. Typically, the actuated arms or fingers apply a compressive force to the perimeter of the wafer to center and capture it.
These mechanical actuation and vacuum-assisted devices and methods of centering and capturing silicon wafers on end effectors are complex from a design standpoint and therefore costly to build and maintain. In addition, the vacuum assisted method may cause backside damage to the wafer and contamination of the other wafers in the cassette because intentional engagement as well as inadvertent touching of the wafer may dislodge particles that can fall and settle onto the other wafers. Wafer backside damage can include scratches as well as metallic and organic contamination of the wafer material.
Mechanically actuated edge grip end effectors minimize the amount of particulate contamination and backside damage to the wafer, but however present difficulties in accurately picking, centering, capturing and transferring the wafer. Mechanically actuated edge grip end effectors also may damage the edge area of the end effector. Moreover mechanical actuators increase the complexity, weight and profile height of the end effector. Therefore end effectors with mechanical actuated wafer grips may be difficult to maneuver through vertical stacking cassettes where the wafers are stored because the separation between wafers is on the order of only about 5-10 mm as specified by SEMI. In addition, the contamination, scratching and damage to the wafers as a result of vacuum assisted and mechanically actuated end effector devices and methods described results in lower production yields and correspondingly higher manufacturing costs.
According, there is a need for an improved end effector coupled to a substrate transport apparatus that decreases contamination and damage to the wafer surfaces, decreases the design complexity of the apparatus, maximizes vertical clearances in the cassette, and more accurately and effectively centers and captures the wafer on the end effector.